WASHINGTON (AP) — The U.S. has given Pakistan nearly $6 billion to pursue terrorists since the Sept. 11 attacks, but with little to no proof that the money has been used for that purpose, an independent audit has found.

The assessment by the Government Accountability Office, released Tuesday, angered members of Congress who say they are concerned that Pakistan — the nation's closest ally in the war on terrorism — is milking the U.S. government.

GAO found, for example, that the U.S. recently gave Pakistan more than $200 million for air defense radars without bothering to investigate whether the money was needed specifically to go after terrorists. Another example included $45 million for road and bunker construction without evidence they were ever built.

"The U.S. government is being asked to reimburse Pakistan for non-incremental air defense radar maintenance when al-Qaida is not even known to have an air force," said Rep. Howard Berman, D-Calif., chairman of the House Foreign Affairs Committee. "The purpose of these funds is to support the fight against extremists, not to boost Pakistan's conventional warfare capability."

Since the 2001 terrorist attacks, the U.S. has given Pakistan some $10.8 billion in aid. About half of the money, $5.6 billion, has been used to reimburse Pakistan for military operations. The country is by far the largest recipient of U.S. military aid, designated "coalition support funds," because of the influx of al-Qaida and Taliban fighters into its unpoliced tribal areas along the Afghan border.